FCC technology has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier, higher molecular weight, less valuable hydrocarbon feed stocks such as gas oil.
In its most general form, the FCC process comprises a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, coke tends to accumulate on the catalyst. Coke must be burned off of the catalyst in a regenerator.
When a catalyst is exposed to oxygenates, such as methanol, to promote a reaction to olefins in a MTO process, carbonaceous material is generated and deposited on the catalyst. Accumulation of coke deposits interferes with the catalyst's ability to promote the MTO reaction. As the amount of coke deposit increases, the catalyst loses activity and less of the feedstock is converted to the desired olefin product. The step of regeneration removes the coke from the catalyst by combustion with oxygen, restoring the catalytic activity of the catalyst. The regenerated catalyst may then be exposed again to oxygenates to promote the conversion to olefins.
Conventional catalyst regenerators typically include a vessel having a spent catalyst inlet, a regenerated catalyst outlet and a combustion gas distributor for supplying air or other oxygen containing gas to the bed of catalyst that resides in the vessel. Cyclone separators remove catalyst entrained in the flue gas before the flue gas exits the regenerator vessel. Downstream vessels which also may utilize cyclonic separation may also be employed to remove catalyst fines from flue gas streams.
The heat of combustion in the regenerator typically produces flue gas at temperatures of 677° to 788° C. (1250° to 1450° F.) and at a pressure range of 138 to 276 kPa (20 to 40 psig). Although the pressure is relatively low, the extremely high temperature, high volume of flue gas from the regenerator contains sufficient kinetic energy to warrant recovery of energy. Flue gas may be fed to a power recovery unit, which may include an expander turbine. The kinetic energy of the flue gas is transferred through blades of the expander to a rotor coupled either to a main air blower, to produce combustion air for the FCC regenerator, and/or to a generator to produce electrical power. The flue gas may also be run to a steam generator for further energy recovery.
Flue gas streams that may have passed through a third stage separator, a flue gas cooler and/or power recovery equipment must still be reduced in pressure before it is exhausted from a stack. Orifice chambers use grids with holes to impose a pressure drop on the flue gas prior to venting through the stack.
More improved pressure reduction vessels can streamline catalyst regenerator flue gas processing.